Taste of Cinema's list description: A hero is only as good as his or her villain. Without compelling enemies, even the best heroic characters tend to feel boring. This is probably because heroes are defined by what they stand for: by representing the opposite of the hero's values, villains help to highlight these values. Quite often, villains are written with even more attention than the protagonist, because they need not only to be "bad", but to be bad in an interesting way.
Straight-up bad guys whiteout nuances in their evil actions are quickly forgotten, and are nothing more than an obstacle in the hero's way. When the villain is particularly charismatic, or even sympathetic in his actions, then you have a great character. This list looks at characters that occupied a villainous role in their film but resonated with the audience and made it actually sympathize with them.